The present invention relates to an arrangement and a method for controlling a combustion engine so that self ignition occurs at an optimum crankshaft angle by control of the compression ratio in the cylinder.
One type of such combustion engines are called HCCI (Homogeneous Charge Compression Ignition) engines and may be regarded as a combination of an Otto engine and a diesel engine. In HCCI engines, a homogeneous mixture of fuel and air is compressed in a combustion chamber until self-ignition of the fuel mixture takes place. Advantages of HCCI engines are that they produce low discharges of nitrogen oxides NOx and soot particles while at the same time being of high efficiency. One reason for HCCI engines not being used conventionally to any great extent is that it is difficult to control the self-ignition of the fuel mixture to a correct crankshaft angle.
Two different valve control strategies are known for controlling under laboratory conditions the self-ignition of the fuel mixture. The first strategy refers to closing the exhaust valve before the combustion chamber has been emptied of exhaust gases from a preceding combustion process and to opening the inlet valve later than usual. Such a so-called negative overlap results in a varying amount of exhaust gases being retained in the combustion chamber for a subsequent combustion process. The hot exhaust gases retained in the combustion chamber raise the temperature of the fuel mixture for the subsequent combustion process. A suitable amount of exhaust gases retained can thus cause the fuel mixture to have an initial temperature such that it self-ignites at a substantially optimum crankshaft angle.
The second strategy refers to controlling the closing of the inlet valve. The compression ratio in the cylinder can be varied by varying the crankshaft angle at which the inlet valve closes. The later the inlet valve closes, the shorter the piston movement required for compression of the fuel mixture. Varying the inlet valve closure and hence the effective compression ratio in the cylinder makes it possible for self-ignition of the fuel mixture to take place at a substantially optimum crankshaft angle.
A disadvantage of the aforementioned strategies is that they provide control of self-ignition in an HCCI engine within a relatively limited load range. Most technical applications need an engine which can be used over a relatively large load range.